The nitrogen of organic fertilizers does not fully mineralize within a season, and hence will partly become available in later years. This effect is taken into account for the first year but generally not in later fertilizer applications. If it would be taken into account, fertilizer use could be more efficient. This study is an analysis of a 13-year field trial where crop yield was measured in 13 fertilizer treatments which differ in total N applied and decomposition rate. This is complemented with a model study in which mineralization and soil nitrogen content were calculated. We intended to show step by step that the use of fertilizers with a low decomposition rate, relative to fertilizers with a high decomposition rate, lead in the course of years to an increase in soil organic nitrogen, an increased nitrogen mineralization and availability and that this results in increased yields. Some of the results in these four steps in our work give support to this, but there are more factors at play. We show that when using, for example, deep stable manure, after some years the available nitrogen may for 50% or more be derived from manure applied in former years. This makes clear that, at least in agricultural systems that use a substantial amount of organic fertilizers, it is worthwhile taking this delayed mineralization into account.